Crosslinkable silyl group-terminated polymers are used as curable compositions for sealants, adhesives, pressure-sensitive adhesives, paint, and potting materials. As the crosslinkable silyl group-terminated polymers, polysiloxane, polyoxypropylene, and polyisobutylene polymers have been known. However, the curable compositions using such polymers have many problems. For example, although polysiloxane polymers exhibit excellent weatherability, heat resistance, and cold temperature resistance, they exhibit unsatisfactory oil resistance, low staining properties, paintability, and gas-barrier properties. Although polyoxypropylene polymers exhibit low staining properties and satisfactory paintability, they have insufficient weatherability. With respect to polyisobutylene polymers, although excellent weatherability, low water vapor transmission, and gas-barrier properties are exhibited, they are difficult to handle due to their high viscosity, and it takes a long time to perform moisture-curing (curing by water).
As moisture-curing compositions which are excellent in heat resistance, weatherability, oil resistance, and low staining properties, and which are easy to handle, curable compositions containing crosslinkable silyl group-terminated acrylic polymers as principal components have been suggested.
Examples of methods for producing acrylic polymers contained in the compositions include methods disclosed in Japanese Examined Patent Application Publication Nos. 3-14068 and 5-72427 in which acrylic monomers are radically polymerized using a crosslinkable silyl group-containing mercaptan chain transfer agent, a crosslinkable silyl group-containing disulfide chain transfer agent, or a crosslinkable silyl group-containing azo polymerization initiator. However, in these production methods, it is difficult to introduce crosslinkable silyl groups into the ends of the polymers reliably, and it is not possible to produce compositions having satisfactory physical properties. Since crosslinkable silyl groups are hydrolyzed, it is not possible to perform water-based polymerization, such as emulsion polymerization or suspension polymerization. Even when solution polymerization is performed, since the water content must be controlled strictly, the production process becomes complicated.
Japanese Examined Patent Application Publication No. 4-55444 discloses a method in which a crosslinkable silyl group-containing hydrosilane or crosslinkable silyl group-containing tetrahalosilane is used as a chain transfer agent. However, in this method, it is also difficult to introduce crosslinkable silyl groups into the ends of the polymer at high yield, and it is not possible to produce cured compositions having satisfactory physical properties. Furthermore, as in the above-mentioned example, since crosslinkable silyl groups are hydrolyzed, it is not possible to perform water-based polymerization, such as emulsion polymerization or suspension polymerization. Even when solution polymerization is performed, since the water content must be controlled strictly, the production process becomes complicated.
Japanese Unexamined Patent Application Publication No. 6-211922 discloses a method in which a hydroxyl-terminated acrylic polymer is produced using a hydroxyl-containing polysulfide chain transfer agent in large excess relative to an initiator, and then the hydroxyl groups are converted into crosslinkable silyl groups. However, in this method, a large amount of chain transfer agent must be used, thus being uneconomical.
In order to overcome these problems, a process is disclosed in Japanese Unexamined Patent Application Publication No. 11-80571 in which a crosslinkable silyl group-terminated vinyl polymer is produced by an atom transfer radical polymerization (ATRP) method using a metal complex as a catalyst. However, in the atom transfer radical polymerization method, since the metal complex is used as the catalyst, purification must be performed after polymerization and the process becomes complicated, resulting in a decrease in productivity. In this method, it is also impossible to employ a water-based polymerization technique, such as emulsion polymerization or suspension polymerization.
On the other hand, reversible addition-fragmentation chain transfer (RAFT) polymerization methods are excellent in producing vinyl copolymers because the molecular weight and the molecular weight distribution are controlled and a wide variety of monomers and a wide variety of polymerization techniques including water-based polymerization can be used. The details thereof including the reaction mechanism are described in PCT Publication No. WO98/01478; PCT Publication No. WO99/05099; PCT Publication No. WO99/31144; Macromolecules, 1998, 31, page 5559; Macromolecules, 1999, 32, page 2071; Macromolecules, 1999, 32, page 6977; Macromolecules, 2000, 33, page 243; etc. However, these documents do not describe a method for introducing crosslinkable silyl groups or do not mention a curable composition. The present invention relates to a process for introducing crosslinkable silyl groups into molecular ends by a RAFT polymerization technique, and a curable composition containing, as an essential component, a crosslinkable silyl group-containing polymer produced by the RAFT polymerization technique.